What is the greatest common factor of $18$ and $20$ ? Another way to say this is: $\operatorname{gcf}(18, 20) = {?}$
The greatest common factor is the largest number that is a factor of both $18$ and $20$ The factors of $18$ are $1$ $2$ $3$ $6$ $9$ , and $18$ The factors of $20$ are $1$ $2$ $4$ $5$ $10$ , and $20$ Thus, the greatest common factor of $18$ and $20$ is $2$. $\operatorname{gcf}(18, 20) = 2$